Modern display systems typically create full-color projected images by projecting three single-color images. The eye of the viewer integrates these single-color images to give the perception of a single, full-color image. Three separate modulators or controllable light sources are often used simultaneously to generate the primary-color images. For example, three liquid crystal display (LCD) panels or digital micromirror device (DMD) arrays and appropriate dichroic filters create three primary-color images using a single light source, or three cathode ray tubes perform both the modulator and light source functions to create three primary-color images.
Alternatively, a single modulator is used with a sequential color light source to sequentially create three primary-color images. If the light source sequences through the primary colors fast enough, the viewer will not see the sequential single-color images but instead will see a single full-color image. A sequential color light source is formed by combining a white-light source with a filtering means such as a color wheel.
A color wheel typically is a disk-shaped assembly of dichroic filters. Other shapes of "color wheels" are possible, such as rotating drums or polygons. The white-light beam is focused on the color wheel and the dichroic filters, which are either transmissive or reflective, filter the white light to form a primary-color light beam. The color wheel includes at least one filter for each of the primary colors.
Spinning the color wheel so that each filter passes through the point at which the white light beam strikes the color wheel generates a sequential primary color light beam. The color wheel typically is spun fast enough to create at least one primary color period for each primary during each frame of a video image. Spinning the wheel faster, or using multiple filter segments for one or more of the primary colors can reduce color separation artifacts that allow the viewer to detect the sequential color nature of the display system.
While a sequential color display system typically costs less to produce than a simultaneous color display system, images created by a sequential filtered color display system are not as bright as images created by simultaneous color display systems using the same light source. This is because at any given time only a portion of the light generated by the light source is being used to form the image. For example, when using a three-color equal-segment filter wheel, each primary color is produced only 1/3 of the time. Additionally, when a sequential filter such as a color wheel is used, the light during the filter transitions, typically called spoke times, will be a varying mixture of the two filters being changed in and out. This mixed-color light is not used since using the mixed color light would adversely affect the color purity of the created image.
High image brightness is one of the major desires of customers in the projection display market. Therefore, a method and system for increasing the brightness of a projected image is needed.